In order to increase the pressure of a compressible fluid, for example, a centrifugal compressor is used. The operation range of a centrifugal compressor may be limited, because surging occurs due to a reverse flow or the like of a fluid while the flow rate thereof is low (when the flow rate of the fluid is decreased in order to increase the pressure of the fluid). When the surging occurs, the operation of the centrifugal compressor becomes unstable. Accordingly, if the surging is suppressed, the operation range of the centrifugal compressor can be extended.
As one means of suppressing surging, casing treatment disclosed in Patent Document 1 is used.
A centrifugal compressor includes an impeller rotating at a high speed, and a casing which accommodates the impeller and in which a scroll passageway is formed around the impeller. In the casing treatment disclosed in Patent Document 1, the wall surface of the casing adjacent to the upstream end of the impeller is provided with a groove formed over the entire circumference of the wall surface, and the groove is communicated with a flow passageway positioned upstream of the impeller. While the flow rate of a fluid is low, a fluid reversely flows upstream of the impeller through the groove from a high-pressure part which locally occurs in an impeller-accommodating portion of the casing, and by recirculating part of fluid, the fluid is prevented from reversely flowing in the impeller-accommodating portion, thereby suppressing the surging.
Using the casing treatment as described above, the effect of suppressing surging is obtained. On the other hand, since a downstream fluid is recirculated upstream, the pressure ratio (the ratio of the suction pressure to the discharge pressure of a compressor) during a low-flow rate is decreased compared to a case where casing treatment is not performed.